OS 10.4 - Please hold power button down....please help!

I have no idea what is going on with my ibook. I would like to say that this is been happening since the last software update but I don't think I can put a finger on it. Every now and then I get this grey screen that has a message saying: Please hold the power button down and shut down the computer....and so on....And this has not happened only once or twice, but several times. The computer completely locks up and nothing except holding down the power button works, as instructed by the screen. Can someone please tell me why this is happening? and the last time this happened, I had just restarted and only Mail and Safari was open, and I was browsing our forums for a solution! Please help me.